AP Chemistry Lab Handout 08 "Identifying an Unknown Hydrogen Carbonate"

Your Name: ____________________ Lab Partner(s):_________________________

Purpose: Use stoichiometry and the Law of Conservation of Mass to

determine the identity of an unknown hydrogen carbonate.

Materials: goggles 150 mL beaker balance

unknown hydrogen carbonate 1.0 M HCl stirring rod

100 mL graduated cylinder crucible w/lid clay triangle

Bunsen burner burner striker ring stand

metal ring clamp tongs wire gauze

Procedure:

Part I: Identification of the Hydrogen Carbonate by Mass Difference

1. Determine the mass of a 150 mL beaker and record.

2. Mass approximately 2.5 g of the hydrogen carbonate in the beaker

and record the total mass of the beaker and the hydrogen

carbonate. Calculate the mass of the hydrogen carbonate and record.

3. Pour approximately 40 mL of 1.0 M HCl into a 100 mL graduated

cylinder, determine the total mass, and record.

4. Slowly pour all the acid into the beaker containing the hydrogen

carbonate. The solution will evolve carbon dioxide. Stir the

solution with a stirring rod until the solid is dissolved and no

additional gas is evolved.

5. Mass the emptied cylinder and record. Subtract the mass of the

emptied cylinder from the mass of the graduated cylinder and the

acid and record. This number is the mass of the acid solution.

Add the mass of the hydrogen carbonate to the mass of the acid solution and record. This number is the mass of the reactants.

6. Mass the beaker containing the reacted solution and record. From

this number, subtract the mass of the beaker and record. This

number is the mass of the products.

7. Subtract the mass of the products from the mass of the reactants

and record. This number is the mass of carbon dioxide evolved.

Calculate the number of moles of carbon dioxide evolved and

record.

8. Calculate the number of moles of hydrogen carbonate and record.

Calculate the molecular weight of the hydrogen carbonate and the

atomic weight of the unknown metal in the hydrogen carbonate and

record.

Part II: Identification of the Hydrogen Carbonate by Decomposition

1. Obtain a clean crucible and dry it for 5 minutes in a flame of

sufficient intensity to cause the bottom of the crucible to glow

red. Let the crucible cool on the clay triangle for 60 seconds.

Using tongs, place the crucible on a piece of wire gauze and allow the crucible to cool to room temperature (requires approximately 5 minutes). Record the mass of the crucible (±0.001 g). From this point forward, handle the crucible with tongs only.

2. Into the crucible, add approximately 2.5 g of the hydrogen

carbonate and record the mass (±0.001 g). Calculate the mass of

the hydrogen carbonate.

3. Put the crucible on a clay triangle supported by a ring and place

the cover slightly off-center. Heat the crucible gently with a

flame for 5 minutes, then increase the flame intensity so that

the crucible glows red and heat for 5 minutes.

4. Let the crucible cool on the clay triangle for 60 seconds. Remove

the lid and allow the crucible to cool to room temperature on a

piece of wire gauze. Record the mass of the crucible (±0.001 g).

5. Calculate the atomic weight of the unknown metal in the hydrogen

carbonate and record.

Results:

Data:

mass of 150 mL beaker:

mass of beaker and hydrogen carbonate:

mass of hydrogen carbonate:

mass of HCl in graduated cylinder:

mass of emptied graduated cylinder:

mass of the HCl solution:

mass of the reactants:

mass of beaker with reacted solution:

mass of products:

mass of carbon dioxide evolved:

mass of crucible:

mass of crucible and carbonate:

mass of carbonate:

mass of crucible with product:

Calculations:

Error Analysis:

Conclusion: Including: The balanced chemical reactions for this lab. Which of the two techniques was most accurate? Support your conclusion with references to your data. Explain any contradictory or inconclusive data and suggest a way to overcome these errors.